Tag: Differentiation

At the end of summer 2017, I gave a talk at the annual meeting of the Deutsche Mineralogische Gesellschaft (DMG) at GeoBremen2017. My talk focussed on the results of my 3-kbar experiments on primtive Icelandic basalts, and how they show that depleted mantle melts are much less likely to survive being processed during their ascent through the crust than enriched melts. In other words, enriched melts are more likely to erupt at the surface and depleted melts are more likely to freeze at depth, fundamentally biasing the record of oceanic magmatism we see at the surface. You can download my slides here.

Experimental liquid lines of descent (LLD) for melts from the depleted Háleyjabunga (Hál) and enriched Stapafell (Sta) eruptions. Grey dots show Icelandic compositions from the Western Volcanic Zone and Reykjanes Peninsula (Shorttle & Maclennan, 2011). The arrow shows where compositions were resynthesised to mimic fractional crystallisation.

In summer 2017, I presented a poster at the excellent IAVCEI Scientific Assembly in Portland. My contribution summarised the findings of my experimental work in Hannover so far. In particular, I focussed the effects of mantle-dervied heterogeneity on the phase equilibria of primitive Icelandic basalts in the 1–7 kbar range. You can download a copy of my poster here.

False-colour backscattered electron image of the run products of an experiment on the Háleyjabunga eruption, Iceland

The environmentally impacting AD 1783–84 Laki eruption was the largest Icelandic eruption to have been directly obseved by humans (Thordarson et al., 1996). However, it is by no means unique in Iceland’s volcanic history: Thordarson & Höskuldsson (2008) note that over 50 eruptions >1 km3 in volume have taken place in Iceland since the end of the last glaciation. The 10 ka Grímsvötn tephra series, or Saksunarvatn Ash, which is distributed across the North Atlantic from Greenland to Germany, is thought to have been generated in a series of large, phreatomagmatic eruptions within the Grímsvötn volcanic zone at the end of the last glacial period (Grönvold et al., 1995; Thordarson, 2014). In this first petrological study of the tephra, we (a team from the universities of Cambridge, Manchester and Iceland) exploited the abundance of primitive crystals and melt inclusions in samples from Lake Hvítárvatn in central Iceland in order to investigate magma evolution and storage processes.

Following the approaches laid out by our recent work on Laki and Skuggafjöll, we defined evolved and primtive macrocryst assemblages in tephra samples, the latter of which was out of equilibrium with the matrix glass and probably derived from disaggregated crystal mushes (e.g., Halldorsson et al., 2008). High-anorthite plagioclase-hosted melt inclusions provided the first direct evidence for the supply of high-Mg#, incompatible trace element-depleted mantle melts to the base of the lithosphere in Iceland’s Eastern Volcanic Zone. Through the critical application of clinopyroxene-melt and melt barometers (Putirka, 2008; Yang et al., 1996) , we suggested that the primtive macrocryst assemblage formed within the mid-crust (4±1.5 kbar) and that the evolved assemblage formed in the shallow crust (<2 kbar) shortly before eruption. We showed, however, that clinopyroxene-melt equilibria are not well calibrated at conditions relevant for the tephra’s pre-eruptive storage. We therefore made the case for further exploration of basalt phase equilibria in the critical 1–7 kbar interval, which is a primary aim of my Humboldt Research Fellowship in Hannover.

Basaltic magmas are often assembled from a diversity of mantle melts that mix and crystallise en route to the Earth’s surface (Sobolev & Shimizu, 1993; Maclennan, 2008). Thus, before any attempt can be made at determining the depths of any pre-eruptive processes, it is essential to understand how melts and and crystals relate to each other.

In this paper, we investigated how the magma that fed the large and environmentally impacting AD 1783–84 Laki eruption was assembled. Olivine-hosted melt inclusion compositions revealed that concurrent mixing and crystallisation of variable mantle melts occurred deep within Laki plumbing system. Indeed, the presence of high-anorthite plagioclase compositions more primitive than any other crystal or melt inclusion composition measured confirmed that the difference components of the Laki lava cannot all be related to the carrier liquid by single liquid line of descent. Furthermore, crystal zonation patterns indicated that multiple crystal mush formation and disaggregation events took place prior to eventual eruption. Combining clinopyroxene-melt barometry with information from crystal textures indicates that most crystallisation took place within the mid-crust, the depth of much recent seismogenic magmatism in the Eastern Volcanic Zone of Iceland (Tarasewicz et al. 2012).

Pantellerites are Fe- and volatile-rich, peralkaline rhyolites that erupt primarily in continental rift settings. As no eruptions of pantelleritic magma have been observed, interpreting the diversity of volcanic phenomena at pantelleritic volcanoes is challenging. Explosive eruptions range in scale from large ignimbrite-forming events like the ~45 ka Green Tuff eruption on Pantelleria to small cone-forming events. Effusive eruptions form structures as diverse as low-aspect-ratio lava domes and high-aspect-ratio lava shields. Although fewer in number than their calcalkaline counterparts, peralkaline rhyolite volcanoes nevertheless present a range of hazards.

The evolution of peralkaline magmas has been the subject of much recent debate, with some authors advocating for pantellerite genesis by melting alkali gabbros (e.g., Avanzinelli et al., 2004), and others favouring extensive fractional crystallisation (e.g., White et al., 2009). The volatile content of pantellerite melts had also been the subject of considerable uncertainty until a recent studied have confirmed the water-rich nature of pantelleritic melts.

In this paper, we presented major element, trace element and volatile compositions from glasses, crystals and melt inclusions from a number of post-Green Tuff eruptions from Pantelleria. The main outcomes were:

A quantification of the degree of aluminous lherzolite melting required to generate the alkali basalts present around northwest coast of Pantelleria (~2%).

A confirmation that pantellerites can be generated by extensive fractional crystallisation (~95%) of alkali basalts.

An evaluation that explosive peralkaline eruptions may emit much more sulphur than metauluminous eruptions of an equivalent size, up to ~100 Mt for a Green Tuff-sized eruption, becasue of the high sulphur solubilty in Fe- and alkali-rich melts.